High-resolution hemispherical reflector antenna

ABSTRACT

A high-resolution antenna provided by numerous antenna elements disposed uniformly over the convex side of a first hemispherical reflector which is subtended by a larger second reflector. The antenna elements are connected through equal length paths to corresponding antenna elements on the concave side of a third hemispherical reflector of a radius equal to that of the first reflector. An input is selectably connected to one of the antenna elements on the first reflector which determines the beam direction from the third reflector. Phase error produced by spherical aberration may be compensated for by means of a planar array.

[ Dec.3,i974

3,305,867 2/1967 Miccioli et 343/1006 Primary ExaminerMaynard R. Wilbur Assistant Examiner-S. C. Buczinski Attorney, Agent, or Firm-James K. Haskell; Robert H. Himes [57] ABSTRACT A high-resolution antenna provided by numerous an- 7/00 ments are connected through equal length paths to corresponding antenna elements on the concave side of a third hemispherical reflector of a radius equal to that of the first reflector. An input is selectably connected to one of the antenna elements on the first reflector which determines the beam direction from the 343/1006 third reflector. Phase error produced by spherical ab- 3/ erration may be compensated for by means of a planar 343/754 may 8 Claims, 12 Drawing Figures Louis Stark, Fullerton, Calif.

Hughes Aircraft Company, Culver City, Calif,

Mar. 2, 1966 References Cited UNITED STATES PATENTS 7 M 7 0 a 09 H 9 REFLECTOR ANTENNA Inventor:

Assignee:

Filed:

Appl. No.: 533,123

Int. Cl. Field of Search waited tates Patent Stark HIGH-RESOLUTION HEMISPHERICAL [52] U.S.Cl.....

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Arron a4 HIGH-RESOLUTION HEMISPHERICAL REFLECTOR ANTENNA This invention relates to a high-resolution hemispherical reflector-type antenna and more particularly to antennas adapted to form'simultaneous multiple beams with minimum orno interaction between the circuits for the different beams and, in addition, adapted to operate over a wide angle of space and over a wide frequency band at microwaves.

The requirement for obtaining simultaneous beams with minimum or no interaction between the circuits for the different beam directions immediately limits the number of basic types of beam forming feeds that can be employed in an antenna system. By way of example, coupling two corporate feeds to the same set of radiating elements produces large interactions within corporate feeds. In general, two basic categories of simultaneous beam formers are applicable; namely, (1 optical systems with multiplefeeds, and (2) microwave beam forming matrices comprised of directional power dividers. The latter category includes Butler matrices and Blass matrices. Although extremely ingenious, the matrix methods require extensive microwave plumbing to provide a narrow beam forming apparatus. In addition, electrical loss, cost and complexity in the matrix type device increases substantially with increasing aperture.

Referring now to contemporary optical systems with multiple feeds, there exists the Luneberg lens and the Gent lens. The Luneberg lens, for the present at least, with regard to achieving a uniform phase front, possesses an extremely difficult fabrication problem for a lens having fractional degree beam width capability. The dielectric loss is also quite large in a Luneberg lens system and may well exceed 6 db in a O.3 beamwidth system. Another fundamental problem concerning feed coupling resides in the Luneberg simultaneous beam former. The effective focal length-to-aperture width ratio is quite short and requires that feeds be spaced no greater'than every half wavelength to achieve a beam separation of l beamwidth or less. This results in a requirement for feeds having small cross-sectional areas whereby substantial feed intercoupling is produced, the result of which is beam intercoupling loss. The Gent lens, on the other hand, while proven as a line source, has possibilities as a pencil beam former provided that the inherent astigmatism of the lens can be reduced to a tolerable level.

It is therefore an object of the invention to provide an improved high-resolution hemispherical reflector beam forming system.

Another object of the invention is to provide a highresolution hemispherical reflector-type antenna capable of forming simultaneous multiple beams over a wide angle of space and over a wide frequency band at microwaves.

Still another object of the invention is to provide a high-resolution hemispherical reflector-type antenna capable of generating simultaneous beams with minimum or no interaction between the circuits corresponding to the different beam directions.

A further object of the invention is to provide a wide angle antenna having an aperture diameter that is substantially less than comparable antennas capable of operating over a comparable wide angle of space and over a comparable wide frequency band at microwaves.

In accordance with the present invention, the highresolution hemispherical reflector-type antenna is provided by an optical reflector system which uses multiple horn feeds to generate a large number of beams. A duplexing array anda transfer array are used to eliminate the problem of aperture blocking, which otherwise would be caused by the large number of feeds. The system is spherically symetric allowing a wide angle of beam scan without deterioration in performance. Since the beams are formed by an optical method, the antenna is a time delay (wide bandwidth) design. The beam former includes a spherical reflector, a spherical duplexing array and a spherical transfer array. The operation of the system can best be described by considering the array operating in a transmitting mode. By reciprocity, the receive beam response pattern is the same as the transmit pattern. A particular transmit horn of the spherical duplexing array is energized in a manner to illuminate a section of the spherical reflector. A wavefront is focused in the return direction at an angle determined by the location of the transmit born. The wave is picked up by the duplexing array and appears at the pickup terminals of this array. The returned wave is decoupled from the beam positioning terminals on the duplexing array by employing orthogonal polarizations. To complete the radiation process, the wavefront from the duplexing array is transmitted along TEM transmission lines to an array having the same curvature as the pickup array thereby radiating a plane wave which ideally has only the phase error attributable to the spherical aberration of the reflector. Any of the horns of the beam positioning array can be used as transmitting horns and no beam degradation due to scanning will result because of the spherical symmetry of the system, i.e., the center of every scanned beam passes through the center of curvature of the spherical transfer array. Hence, there is a stationary planar radiating aperture for this system, which passes through the center.

In addition to the above, planar arrays may be added to the antenna system by placing low noise amplifiers in between corresponding elements on the planar array surfaces thereby to reduce the noise temperature of the antenna system by overcoming the beam forming losses. Moreover, additional tapering may be provided across the aperture of the antenna system by tapering the gain of the amplifiers in the planar arrays. As an additional feature, the use of planar arrays permits compensation of phase error produced by spherical aberration. This will produce a basic antenna pattern for all beam positioning horns which possess lower side lobes.

The above-mentioned and other features and objects of this invention and the manner of obtaining them will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a schematic circuit diagram of the high resolution hemispherical-reflector antenna of the present invention;

FIG. 2 shows a partial sectional view of the beampositioning and pickup array in the antenna apparatus of FIG. 1;

FIG. 3 shows a preferred arrangement of the beampositioning horns in the apparatus of FIG. 2;

FIG. 4 shows a partial sectional view of the transfer array in the antenna apparatus of FIG. 1;

FIG. 5 illustrates a four-horn cluster for beampositioning and pickup functions;

FIGS. 6 11 illustrate the details of the horns in the apparatus of FIGS. 2, 4 and 5; and

FIG. 12 illustrates phase variation of reflector sphere for various values of focal length.

Referring now to FIG. 1 of the drawings, there is shown a schematic diagram of the high resolution hemispherical reflector antenna of the present invention. In particular, the antenna includes a spherical reflector 10 which has a radius of the order of 37.5 feet about a point A disposed on a dashed line 11 which designates the center line, i.e., the line of symmetry through the reflector 10. In addition, somewhat less than a hemisphere is employed making the diameter of reflector of the order of 71.8 feet. Next, a spherical duplexing array 12 with a diameter of the order of 37 feet and a radius of the order of 19.3 feet about the point A is disposed symmetrically about the center line 11 with the convex side thereof within the concave of reflector 10, whereby the edges of the reflector 10 and the spherical duplexing array 12 terminate in a common plane that is normal to the center line 11. The spherical duplexing array 12 includes 176,000 horns 14 (shown in FIG. 2) which horns 14 are directed outwards towards the reflector 10. A portion of the horns 14 are connected to a switching matrix, not shown, through 28,500 coaxial lines 16 in a manner hereinafter explained.

A spherical transfer array 18 of the same radius but with center at point B on the center line 11 and with a diameter the same as the spherical duplexing array 12 is preferably but not necessarily disposed symmetrically about the center line 11 and spaced from the duplexing array 12 with the convex side thereof facing the concave side of the duplexing array 12. The spherical transfer array 18 includes 176,000 horns disposed in the same pattern as the array of horns 14 in the duplexing array 12. Unlike the horns 14 in the duplexing array 12 which faced outwards toward the'refiector 10, the horns 15 of the transfer array 18 are directed inwards towards the center of the concave side of the spherical transfer array 18. The horns 14 of the duplexing array 12 are connected to corresponding horns of the transfer array 18 by equal length coaxial lines 20.

Lastly, a circular planar transfer array 22 may be disposed symmetrically about the center line 11 transversely through the point B (assuming the transfer array 18 is symmetrically disposed about center line 11). The circular planar transfer array 22 includes, for example, 64,000 elements which face the concave side of transfer array 18 and is of the order of 22 feet in diameter. A circular planar receive array 24 is then preferably but not necessarily disposed symmetrically about center line 11 and spaced from the transfer array 22. Receive array 24 includes a corresponding number of elements, i.e., 64,000 elements, which elements are arranged in a pattern that is the same as that of the elements of the transfer array 22. The 64,000 elements of the transfer array 22 are then connected to the corresponding elements of the receive array 24 through 64,000 individual tunnel diode preamplifiers 26. The overall diameter of the receive array 24 is the same as that of the transfer array 22.

It should be noted that the lens portion of the highresolution hemispherical reflector antenna of the invention, in some applications, can radiate directly so that the planar arrays 22, 24 may be omitted. The planar arrays 22, 24, however, provide a tool for sidelobe control through tapering and as a location for low noise amplifiers 26 to overcome the loss of the beam forming and switching system.

Thus the high-resolution hemispherical reflector an tenna of the invention includes several assemblies, namely, the spherical reflector 10, the duplexing array 12, the transfer array 18, the transfer and receive circular arrays 22, 24, and a supporting structure, not shown. Each assembly is properly aligned and attached to the supporting structure by conventional means. In locating the assemblies l0, 12, 18, 22, 24, it may be advantageous to employ pinning" so as to provide a means for reassembling the antenna assemblies without the need for realignment. In addition, each individual tunnel diode preamplifier 26 connecting the elements of the receive array 24 to the respective elements of the transfer array 22 is preformed with coaxial cable connectors, prior to being installed in the antenna.

The spherical reflector 10 may be supported in a con ventional manner by means of a compression ring 27 (FIG. 1) and a truncated cone support 28 attached from the periphery of the reflector 10 to a metal mounting plate 30. The reflector 10 may be fabricated as one integral unit by means of a lay-up of laminated glass cloth impregnated with epoxy resin.'ln this case, a rigid polyurethane foam core is used to serve as a mold to provide a support surface for the impregnated glass laminate during wet lay-up, and also to provide stiffness to the unit. The epoxy surface of the reflector is first sandblasted. Then a coating of flame sprayed zinc is applied, followed by a coating of flame sprayed aluminum, for a total metal thickness of between 0.010 and 0.018 inch.

Referring to FIGS. 2 and 4, the spherical duplexing and transfer arrays 12, 18 are identical in construction except for the difference in the location and positioning of the horns l4 and 15. The arrays 12, 18 are fabricated by bonding aluminum spherical dishes 32, 34 to rigid polyurethane cores 36, 38, respectively and riveting aluminum skirts 40, 42 to the outer flanges of the spherical dishes 32, 34. The horns 14, 15 may be arranged in the pattern shown in FIG. 3 wherein the horns 14 or 15 form pentagons which successively increase in size by one additional horn in each side. In general, the spacing between adjacent horns 14, 15 is sufficiently small so as to avoid high reflection losses due to the formation of higher order scattering lobes.

A spacing of one-half a wavelength has been found to give satisfactory performance.

Similarly, the circular transfer and receive arrays 22, 24 are fabricated by bonding two aluminum ground planes to a rigid polyurethane core (not shown) thereby to assure proper ground plane spacing and provide additional support for the elements of these arrays. The circumferences of the arrays 22, 24 may, for example, be reinforced with rolled channel compression rings.

Referring now to FIGS. 6 l 1, there is shown a manner in which the horns 14, having orthogonal polarization properties, may be fabricated. The orthogonal polarization properties of the horns 14 is necessary so as to decouple the wave returned from the spherical reflector from the switching matrix connected to the horns 14 through the coaxial cables 16. In particular,

the horns 14, of FIGS. 6 11, achieve orthogonal polarization properties by transmitting and receiving opposite screw senses of circular polarization. Referring to FIG. 6, the horns 14 include a metal cylinder 40 having metal ridges 41, 42, 43, 44 disposed in quadrature along the entire length thereof thereby forming a quadruple ridge-loaded circular waveguide. The metal ridges 41, 43 and 42, 44 have dielectric spacing elements or slabs 45 placed across opposite ridges 41, 43 and 42, 44 to provide optimum phase shift. Broadband impedance matching is achieved by stepping the dielectric at both ends of the slabs 45. The material used for the slabs 45 is a low loss dielectric. In addition, ridges 41 44 are tapered to accommodate a horn element 46, FIGS. 8, 11, at one extremity of the metal cylinder 40. The horn element 46 has a circular aperture through the center portion thereof having a diameter less than the spacing between the metal ridges 42, 44 and 41, 43. The extremity of metal cylinder 41) opposite the horn element 46 is capped with a metal disc 47, FIGS. 9, 10, which supports orthogonal loop couplers 48, 49 which are inserted into metal cylinder 40 in a manner to couple to orthogonal regions defined by ridges 41 44. An extremity of each loop coupler 48, 49 extends through the disc 47 and connects to the center conductors of coaxial connectors 50, 51. I

Thus, a signal applied to coaxial connector 50 radiates from horn element 46 as a circularly polarized wave of predetermined screw sense. Upon being reflected from a conductive surface, the screw sense of the wave reverses whereby the wave received by horn element 46 appears at coaxial connector 51. In assembling the antenna system, the coaxial lines 26 are connected to inputs or horns 14 which result in waves of the same polarization radiating from horn elements 46 thereof. The coaxial input 50 or 51 to a horn 14 need not be used if it is desired to standardize on the same type of horn 14 for the horns l5 and for the horns 14 not connected to the coaxial line, 16, i.e., horns 14 not used for a discrete beam direction. In addition, one coaxial line 16 may be branched in a manner to connect to a cluster of four horns 14, FIG. 5, for beam positioning requirements hereinafter explained. In view of the large number of horns required for the spherical and circular arrays 10, 12, 18, 22, 24, an impact extrusion process may be employed which produces a quadruple ridged circular waveguide in a relatively simple operation. The measured power loss in a l4-inch-long, impact-extruded tube is only 0.06 db. Also horn elements 46 of the horns 14 may be covered with individual radomes if it is not desired to enclose the overall antenna assembly in a large radorne.

The operation of the antenna system can be best described by considering the duplexing array 12 operating in a transmitting mode. By reciprocity, the receive beam response pattern is the same as the transmit pattern. A particular transmitting horn, T FIG. 1, is energized and illuminates a section of the spherical reflector 10. A wavefront is focused in the return direction, 6,. The wave is picked up by the duplexing array 12 and appears at the pickup terminals of this array.

The returned wave is decoupled from the beam positioning terminals on the duplexing array 12 by employingorthogonal polarizations. To complete the radiation process, the wavefront from the duplexing array 12 is sent along TEM transmission lines 20 to transfer array 18 having the same curvature as the duplexing array 12. A plane wave is therefore radiated which ideally has only the phase error attributable to the spherical aberration of the spherical reflector 10. Any of the horns 14 of the beam positioning or duplexing array 12 can be used as transmitting horns and no beam degradation due to scanning will result because of the spherical symmetry of the system. The center of every scanned beam passes through the center of curvature of the spherical transfer array 18 as shown in FIG. 1. Hence, there is a stationary planar radiating aperture for this system, which passes through the center.

There are three basic methods using orthogonal polarizations which may be used to decouple the wave returned from the spherical reflector 10 from the beam positioning coaxial inputs 16. In the first method, the duplexing horns 14 transmit and receive opposite screw senses of circular polarization. This method uses the normal properties of a smooth reflecting surface to reverse the screw sense. In a second method, duplexing horns would transmit and receive crossed linearly polarized signals. This method requires a grating tilted at 45 to the incident polarization in front of the reflector surface to produce the required rotation of the incident polarization. A third method is possible which is nonreciprocal. In this method, a horn would have a single terminal which is driven from a three-port circulator. This latter method is suitable for a receive only system. The first of these three methods is preferred and ac,- cordingly is described above as the preferred embodiment.

Optical design of the spherical reflector 10 is based upon the desire to obtain a minimum reflector size consistent with the beamwidth and maximum phase error requirements. The spherical duplexing array 12 and transfer array 18 are arrays on spherical surfaces of radius approximately one-half the radius of the spherical reflector 10. Minimizing the reflector size reduces the area of the arrays and reduces the number of array elements or horns. The design of the spherical reflector follows from choosing the'best location of a feed point with respect to the spherical reflector 10. Then the spherical aberration is either minimized or set up for best compensation at a final planar aperture, if such is used.

The geometry is shown in FIG. 12(a). The focal length, f, is defined as the distance between the feed point and the surface of spherical reflector 10. If the phase variation (path length error, A) is examined along a line through the center of the reflector sphere, as shown in FIG. 12(a) for various values of the focal length, f, the set of curves of FIG. 12(b) are obtained. It can be shown that for all values off greater than or equal to R/2, (i.e., one-half the radius of spherical reflector 10) the phase error increases monotonically from the center of the aperture to the edges. Alternately, for any value off less than R/2, the phase error will have a positive maximum and a zero on either side of the center as shown in FIG. 12(b).

The purpose of the spherical duplexing array 12 is to provide a set of beam positioning horns l4 and also to couple, essentially unchanged, the focused plane wavefront from the spherical reflector 10 (viewing the radiation process as one of transmit) to the transfer array 18. In the beam positioning function, the horns 14 must provide an optimum illumination taper on the reflector 10, and in addition, the spacing between horns should be close enough to provide sufficient beam gain at cross over. In the other duplexing function, i.e., when receiving the focused plane wave, the array elements of the duplexing array 12 must not produce a varying amplitude or phase across the spherical array. The extent to which phase and amplitude distortion occurs can be characterized by the complex active element pattern of the elements on the spherical array.

The principal loss factors in beam forming, excluding ohmic loss, are the beam cross coupling loss and the spillover loss. Another smaller factor affecting loss is the mismatch of the spherical duplexing array 12 to the wavefront from the reflector It). All these loss factors are related to each other through the mutual coupling effects between the horns in the array. The amount of mutual coupling between the horns is controlled by the element spacing, the type of elements (linearly or circularly polarized), and the aperture ground plane spacing of the elements. A brief description of the beam forming losses and the methods of minimizing these losses is given below.

Cross Coupling and Spillover Losses The problem of cross coupling between beams in multiple beam antennas and the limitations imposed by the cross coupling on the array gain have been studied extensively. In general, the cross coupling between beams is determined by the cross-correlation of the radiation patterns corresponding to these beams. This cross-correlation is defined by a set of beam coupling factors:

closed surface including spillover region in which E (6, (b) the array pattern of the kth beam;

E, (6, (b) the array pattern of the )th beam;

* denotes complex conjugate; and

6 and (b are the usual spherical coordinates.

When this set of beam coupling factors vanishes identically, the cross coupling loss is zero. In general, some of these beam coupling factors are finite and effect a loss in antenna gain. Physically, the finiteness of a beam coupling factor has the meaning that the radiated power contained in two beams energized simultaneously is different from the sum of the radiated powers contained in the individual beams when each of the beams operates separately. This is indicative of the fact that the driving point impedance as seen in one feed terminal has been changed by the presence of excitation in the other feed terminal; that is, there is coupling from one feed terminal to another. Another way of viewing this phenomenon is to say that the effective apertures of the beam positioning horns l4 overlap when the corresponding beams couple to each other.

In order to eliminate the cross coupling between beams, the secondary patterns corresponding to these beams must be orthogonal to each other- When this happens the effective aperture of each beam positioning horn l4 equals the physical area corresponding to the element spacing in the array; hence, the effective apertures or the horns just border each other without overlapping. Corresponding to this condition, the only loss in the antenna is the spillover loss determined by the active element pattern corresponding to the above effective aperture.

The spillover contributions to the cross-correlation integral always reinforce at the dish edge, but the total contribution tends to vanish, once a wavelength or so of feed separation is achieved, due to the separation of phase centers. Thus, beam separation for orthogonality will take place at approximately I beamwidth of separation, as in the case of no spillover, provided sufficiently large feed separation is used. If feed separation is small, it may never be possible to position the secondary beams for orthogonality.

For a 35 db Taylor aperture distribution in the circular planar array 22, the horn spacing in the spherical arrays l2, 18 in wavelengths is given in terms of the beam spacing in beamwidths by the relation d/)\ 1.26 (G /3W) (f/D), where 0, is the beam separation, BW is the beamwidth and f/D relates to the spherical reflector 10, as previously defined. In order to provide orthogonality between beams, the angular spacing between beams must be approximately I beamwidth. Corresponding to this beam separation and an f/D of unity, which is approximately correct for a beamwidth of O.3, the spacing between the horns in the beam positioning array 12 is approximately I wavelength. This spacing is too large for the spherical array 12 to efficiently perform its function of coupling to a plane wavefront. This situation is made evident from grating lobe considerations. Therefore the elements are spaced a half wavelength on the array and used four at a time in the beam positioning function and singly in the other function. This arrangement of the horns 14 is shown in FIG. 5. Cross coupling loss between blocks of four horns is extremely small, and the main contribution to loss becomes spillover. Spillover in an arrangement such as this might typically be 2 db.

Reflection Losses of the Spherical Duplexing Array 12 The efficiency of the spherical duplexing array 12 in receiving the focused wavefront from the spherical reflector It) can be analyzed in terms of the active element pattern of the array horns 14. If the active element pattern is G (4'n'A/A cos 6,,

where 0,, is the angle measured from the normal of the element and A is the area allotted to each element, then the element will be matched for all angles of incidence and capture all the available power. This analysis is completely rigorous for a planar array and for spherical arrays when the radius of curvature is large.

If all coaxial lines 20 leading to the spherical transfer array 18 were energized equally, then a uniform amplitude wavefront would ,be radiated, assuming the ideal active element pattern having a cos 0,, variation is achieved. The cos 0,, tapering effect of the element pattern is exactly cancelled by an inverse space taper to give a uniform amplitude when projected on a plane as in the case with equal signals feeding all horns 15 of the transfer array 18. Higher sidelobes than the customary, i.e., 17 db, will be encountered in this case because of the tilted elements forcing more power in the sidelobe directions. In actual operation, the active element patterns of the horns 14 of the duplexing array 12 impress a cosine variation on the signals received. Thus the radiated wavefront has a cosine amplitude taper whose edge value is determined by the tilt of the end horns of the effective aperture. Assuming the angular aperture of the spherical arrays l2, l8 lies between 45, the edge taper would be 15 db.

In general, the shape of the active element pattern is mainly controlled by the spacing of the horns (in terms of wavelength) and the height of the horns above a ground plane. The extent of the influence of neighboring horns on a given horn depends upon how fast the mutual coupling between the horns falls off with respect to horn separation. For a uniformly spaced array with the elements'matched for broadside direction, the 3 db beamwidth of the active element pattern will tend to get narrower as the element spacing increases. By way of example, in an experimental situation where the choke depth of the horns was chosen so that the driving point impedance of each horn was matched for the broadside direction, the 3 db beamwidth of the active element patterns was reduced from 67 to 27 when the horn spacing was increased from 0.71% to 0.79) ln general, for an element spacing greater than 0.5 wavelength, the 3 db beamwidth of the active element pattern corresponds roughly to the angle, 26 over which the array can be scanned without grating lobe formation, as given by the well known relation S/A Sin mur) In order to approach the ideal active element pattern in the spherical array 12, a small area of the array can be examined. This small area of the spherical array 12 can be flattened to a planar array without loss of accuracy. Then, in this simplified configuration, the horn projection above the ground plane can be varied to obtain the closest approximation to the ideal gain function, viz.,

G (41rA/A cos 6 The electrical spacing may also be varied by changing the frequency at which the patterns are recorded.

Spherical Transfer Array 18 The design of the spherical transfer array 18 is largely fixed once the duplexing array 12 has been determined. The design is based upon the requirements of passing the incident wavefront through the spherical arrays without any distortion and providing good aperture match. ln order to meet these requirements, the surface of phase centers of the transfer array 18 must be identical to that of the duplexing array 12. Since the transfer array 18 is concave while the duplexing array 12 is convex, a slightly different horn-ground plane spacing adjustment produces the optimum active element pattern in each case.

The size of the spherical duplexing array 12 and transfer array 18 must be large enough to fully illuminate the circular planar arrays 22, 24 for the maximum scan angle. The diameter, L, of the transfer array 18 is given by the following relation:

L 2r sin [0,, sin (D/2r) cos G where r radius of the transfer array 18;

D diameter of the circular planar array 22; and

6 maximum scan angle.

Horn Design The duplexing horn 14 design is based upon three requirements. It should provide (1) at least 20 db isolation between the beam positioning and transfer terminals, i.e., the coaxial connectors 50, 51, FIG. (2) an isolated element pattern which is broad and symmetrical about its longitudinal axis (the active element pattern in the array should follow as closely as possible to a cosine shape); and (3) a polarization of the radiated signal from the horn which is constant, independent of angle. The duplexing horn M using dual circular polar ization which possesses these requirements was described in connection with FIGS. 6 ill. The horns 14 may, of course, be used in all of the situations where horns are required so long as the polarization is maintained consistent.

Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in theart that various changes in form and arrangement of parts may be made to suit requirements without department from the spirit and scope of the invention.

What is claimed is: v

1. A lens antenna adapted to operate over a wide range of frequencies, said antenna comprising a first spherical conductive shell subtending a predetermined solid angle having a first center of curvature, the radius of said first shell being large compared to a wavelength at the lowest frequency in said wide range of frequencies; a second spherical conductive shell having said first center of curvature and subtending a solid angle that is at least coextensive with said predetermined solid angle, the radius of said second shell being no more than one-half the radius of said first shell; a third spherical conductive shell having a second center of curvature spaced from said first center of curvature and a radius of curvature equal to the radius of curvature of said second shell; means including a plurality of antenna elements disposed over the convex side of said second shell for directing an electromagnetic wave of predetermined polarization towards the inner surface of said first shell from a predetermined location on said second shell and for receiving energy reflected from said first shell of a polarization orthogonal to said predetermined polarization at locations corresponding to each antenna element of said plurality thereof on said second shell; a corresponding plurality of antenna elements disposed in corresponding positions on the concave side of said third shell; and means connecting respective antenna elements on said second shell to corresponding antenna elements on said third shell for transmitting said reflected energy thereto over paths of equal electrical length.

2. A lens antenna adapted to operate over a wide range of frequencies, said antenna comprising a first conductive shell subtending a predetermined solid angle no greater than a hemisphere and having a uniform predetermined radius of curvature as measured from a first center of curvature, said uniform predetermined radius being large compared to a wavelength at the lowest frequency in said wide range of frequencies; a second conductive shell having a uniform radius of curvature as measured from said first center of curvature that is equal to no more than one-half said predetermined radius and subtending a solid angle that is coextensive with said predetermined solid angle; a third conductive shell subtending a solid angle equal to said predetermined solid angle and having a uniform radius of curvature about a second center of curvature spaced from said first center of curvature, said radius of curvature of said third shell being equal to said radius of curvature of said second shell; means for providing respective corresponding antenna elements over the convex side of said second shell and the concave side of said llll third shell, said antenna elements on said second shell being adapted to transmit electromagnetic energy of predetermined circular polarization to respective corresponding antenna elements on said third shell over equal length electrical paths; and means coupled to one of said antenna elements on said second shell for causing electromagnetic energy of a circular polarization opposite to said predetermined circular polarization to be radiated towards said first shell thereby to cause a beam to be radiated in a unique direction from the antenna elements on saidthird shell.

3. The lens antenna adapted to operate over a wide range of frequencies as defined in claim 2 wherein each of said one antenna elements comprises a length of ridge-loaded circular waveguide, a conductive horn having an aperture through the center portion thereof disposed across one extremity of said length of ridgeloaded circular waveguide, a conductive cap having first and second orthogonal coupling loops disposed across the remaining extremity of said length of ridgeloaded circular waveguide, said first and second coupling loops coupling to diametrically opposite portions of said circular waveguide intermediate the ridges thereof, and first and second coaxial connectors connected to said first and second coupling loops, respectively.

4. The lens antenna adapted to operate over a wide range of frequencies as defined in claim 2 wherein the respective ones of said plurality of antenna elements on the convex side of said second shell and said corresponding plurality of antenna elements on the concave side of said third shell have a separation that is substantially one-half wavelength at the highest frequency within said wide range of frequencies.

5. The lens antenna adapted to operate over a wide range of frequencies as defined in claim 2 wherein said first, second and third shells have a common axis of rotation.

6. A lens antenna adapted to operate over a wide range of frequencies, said antenna comprising a first conductive shell subtending a predetermined solid angle no greater'than a hemisphere and having a uniform predetermined radius of curvature as measured from a first center of curvature, said uniform predetermined radius being large compared to the wavelength of the lowest frequency in said wide range of frequencies; a second conductive shell having a uniform radius of curvature as measured from said first center of curvature that is equal to no more than one-half said predetermined radius and subtending a solid angle that is coextensive with said predetermined solid angle; a third conductive shell subtending a solid angle equal to said predetermined solid angle andhaving a uniform radius of curvature about a second center of curvature spaced from said first center of curvature, said radius of curvature of said third shell being equal to said radius of curvature of said second shell; means for providing antenna elements over the convex side of said second shell and corresponding antenna elements over the concave side of said third shell, said antenna elements on said second shell being adapted to transmit electromagnetic energy of predetermined circular polarization to respective corresponding antenna elements on said third shell over paths of equal electrical length; means coupled to one of said antenna elements on said second shell for causing electromagnetic energy of a circular polarization opposite to said predetermined circular polarization to be radiated towards said first shell; 21 first planar array disposed through said second center of curvature normal to the axis of rotation of and facing said third shell; and a second planar array similar to said first planar array, the respective elements of said first planar array being connected to corresponding elements of said second planar array whereby a beam is radiated in a unique direction therefrom as determined by the position of said one of said antenna elements on said second shell.

7. The lens antenna adapted to operate over a wide range of frequencies as defined in claim 6 wherein the radius of curvature of said second and third shells is one-half the radius of said first shell, said first and second planar arrays are circular and have respective diameters that are comparable with or greater than the radius of curvature of said third shell and said first, second and third conductive shells and said first and second circular planar arrays have a common axis of rotation.

8. The lens antenna adapted to operate over a wide range of frequencies as defined in claim 6 wherein amplifiers of equal phase delay are interconnected between respective corresponding elements of said first 

1. A lens antenna adapted to operate over a wide range of frequencies, said antenna comprising a first spherical conductive shell subtending a predetermined solid angle having a first center of curvature, the radius of said first shell being large compared to a wavelength at the lowest frequency in said wide range of frequencies; a second spherical conductive shell having said first center of curvature and subtending a solid angle that is at least coextensive with said predetermined solid angle, the radius of said second shell being no more than one-half the radius of said first shell; a third spherical conductive shell having a second center of curvature spaced from said first center of curvature and a radius of curvature equal to the radius of curvature of said second shell; means including a plurality of antenna elements disposed over the convex side of said second shell for directing an electromagnetic wave of predetermined polarization towards the inner surface of said first shell from a predetermined location on said second shell and for receiving energy reflected from said first shell of a polarization orthogonal to said predetermined polarization at locations corresponding to each antenna element of said plurality thereof on said second shell; a corresponding plurality of antenna elements disposed in corresponding positions on the concave side of said third shell; and means connecting respective antenna elements on said second shell to corresponding antenna elements on said third shell for transmitting said reflected energy thereto over paths of equal electrical length.
 2. A lens antenna adapted to operate over a wide range of frequencies, said antenna comprising a first conductive shell subtending a predetermined solid angle no greater than a hemisphere and having a uniform predetermined radius of curvature as measured from a first center of curvature, said uniform predetermined radius being large compared to a wavelength at the lowest frequency in said wide range of frequencies; a second conductive shell having a uniform radius of curvature as measured from said first center of curvature that is equal to no more than one-half said predetermined radius and subtending a solid angle that is coextensive with said predetermined solid angle; a third conductive shell subtending a solid angle equal to said predetermined solid angle and having a uniform radius of curvature about a second center of curvature spaced from said first center of curvature, said radius of curvature of said third shell being equal to said radius of curvature of said second shell; means for providing respective corresponding antenna elements over the convex side of said second shell and the concave side of said third shell, said antenna elements on said second shell being adapted to transmit electromagnetic energy of predetermined circular polarization to respective corresponding antenna elements on said third shell over equal length electrical paths; and means coupled to one of said antenna elements on said second shell for causing electromagnetic energy of a circular polarization opposite to said predetermined circular polarization to be radiated towards said first shell thereby to cause a beam to be radiated in a unique direction from the antenna elements on said third shell.
 3. The lens antenna adapted to operate over a wide range of frequencies as defined in claim 2 wherein each of said one antenna elements comprises a length of ridge-loaded circular waveguide, a conductive horn having an aperture through the center portion thereof disposed across one extremity of said length of ridge-loaded circular waveguide, a conductive cap having first and second orthogonal coupling loops disposed across the remaining extremity of said length of ridge-loaded circular waveguide, said first and second coupling loops coupling to diametrically opposite portions of said circular waveguide intermediate the ridges thereof, and first and second coaxial connectors connected to said first and second coupling loops, respectively.
 4. The lens antenna adapted to operate over a wide range of frequencies as defined in claim 2 wherein the respective ones of said plurality of antenna elements on the convex side of said second shell and said corresponding plurality of antenna elements on the concave side of said third shell have a separation that is substantially one-half wavelength at the highest frequency within said wide range of frequencies.
 5. The lens antenna adapted to operate over a wide range of frequencies as defined in claim 2 wherein said first, second and third shells have a common axis of rotation.
 6. A lens antenna adapted to operate over a wide range of frequencies, said antenna comprising a first conductive shell subtending a predetermined solid angle no greater than a hemisphere and having a uniform predetermined radius of curvature as measured from a first center of curvature, said uniform predetermined radius being large compared to the wavelength of the lowest frequency in said wide range of frequencies; a second conductive shell having a uniform radius of curvature as measured from said first center of curvature that is equal to no more than one-half said predetermined radius and subtending a solid angle that is coextensive with said predetermined solid angle; a third conductive shell subtending a solid angle equal to said predetermined solid angle and having a uniform radius of curvature about a second center of curvature spaced from said first center of curvature, said radius of curvature of said third shell being equal to said radius of curvature of said second shell; means for providing antenna elements over the convex side of said second shell and corresponding antenna elements over the concave side of said third shell, said antenna elements on said second shell being adapted to transmit electromagnetic energy of predetermined circular polarization to respective corresponding antenna elements on said third shell over paths of equal electrical length; means coupled to one of said antenna elements on said second shell for causing electromagnetic energy of a circular polarization opposite to said predetermined circular polarization to be radiated towards said first shell; a first planar array disposed through said second center of curvature normal to the axis of rotation of and facing said third shell; and a second planar array similar to said first planar array, the respective elements of said first planar array being connected to coRresponding elements of said second planar array whereby a beam is radiated in a unique direction therefrom as determined by the position of said one of said antenna elements on said second shell.
 7. The lens antenna adapted to operate over a wide range of frequencies as defined in claim 6 wherein the radius of curvature of said second and third shells is one-half the radius of said first shell, said first and second planar arrays are circular and have respective diameters that are comparable with or greater than the radius of curvature of said third shell and said first, second and third conductive shells and said first and second circular planar arrays have a common axis of rotation.
 8. The lens antenna adapted to operate over a wide range of frequencies as defined in claim 6 wherein amplifiers of equal phase delay are interconnected between respective corresponding elements of said first and second planar arrays. 